Highlights

“I think there’s a general sense within the forest community that we’re seeing the beginning of the loss … of sugar maples,” said Rock, a professor of forestry and botany at the University of New Hampshire, who has studied New England’s woodlands for decades.

He attributes the maple trees’ problems largely to the effects of climate change that is worsened by human activity and says it is was predicted years ago but is occurring faster than expected.

That loss is most obviously seen, Rock says, by a dimming in the brilliance of the sugar maple’s fall foliage over the past three years. The key point is that he means “seen” in a scientific sense, based on three years of data about such things as foliage color measured by spectral analysis of satellite photos.“This isn’t my saying so, because I think they look good, this is the numbers saying so,” he said.Some of those numbers are prompted by work of Martha Carlson, a Ph.D. candidate under Rock who, with her husband, owns a Center Sandwich farm that has a 60-acre “sugarbush” operation.

Carlson, a former journalist and teacher, enrolled at UNH because she was worried about the condition of maples on her farm and elsewhere, including a decline in sweetness of the sap.

Sugar levels of 3 percent used to be not uncommon, she says. Now, similar trees have sugar levels around 1 percent.

“Imagine if you have 3 percent something in your blood and it drops to 1 percent – you’d probably be in the hospital,” she said.“This seems to be a real phenomenon that I don’t know that anyone has documented before,” Rock said.

The drop in sugar levels is directly connected to a loss in autumn brilliance, causing a decline in anthocyanin – a chemical that creates the color red. Less sugar leads to less anthocyanin, which means less red to combine with yellow to create orange.Different colors reflect different wavelengths of light. Measuring those wavelengths, doing spectral analysis, turns subjective opinions about “leaf-peeping” pleasure into objective data. That data, says Rock, tells the story of how one of the region’s premier moments is being diluted.

“You get numbers that tell you how red the foliage is, how orange the foliage is,” he said. “Since 2003, we just haven’t had a typical fall color display. They’ve been muted, they’ve been delayed.”

As far as what’s going on inside the trees, Carlson’s main work is to find and calibrate easily measurable items – “four or five thermometers, if you will, to measure the health of sugar maples,” as Rock puts it.

“I want to see if I could see the decline of the maples and also help teachers and sugar producers, ordinary citizens, to learn how we see if climate change is here and what it’s doing to our forest,” Carlson said.

“We are working to develop some very simple measurements of things that anybody can see, like timing of when the leaves open – called bud burst – and leaf size and leaf retention, how long do they stay on the tree … and the quality of the sap season,” she said.

Signs of stress

UNH professor Barrett Rock points to five indicators that the region’s sugar maples are under stress:

Muted fall displays over the past seven years. Causes include low sugar content in the leaves prior to the onset of the fall color display, and lack of early frosts in September due to increasingly longer growing seasons and other abnormal weather conditions (torrential rains, high winds, etc.) Both of these factors are likely due to recent temperature increases in the region of nearly 2 degrees Fahrenheit since 1899.

Low levels of chlorophyll prior to the onset of fall colors.

Decreased levels of sugar in springtime sap as determined by the number of gallons of sap needed to make a gallon of syrup. In the past it took between 32 and 35 gallons of sap to make a gallon of syrup, while it now takes between 45 and 50 gallons of sap to make a gallon of syrup.

Cellular levels of damage in foliage across the growing season, resulting in low moisture contents in leaves and canopies as measured by field spectrometry and wet weight/dry weight measurements.

Increased occurrence of fungal leaf pathogens, apparently due to milder weather allowing over-wintering of fungal spores. These fungal diseases reduce the amount of sugar left in the leaves prior to leaf drop.

A note about identifying climate change impacts

Many events are linked to climate change by virtue of fitting a long-term trend. For example, the record-breaking “once in one thousand years” rains that drove the recent Nashville flood is part of the long-term trend of increasing heavy precipitation events in the Southeastern United States that has been fully documented and firmly attributed to climate change. While one cannot say with certainly that event was "caused" by climate change, one can and should say it is linked to climate change.